1. Field of the Invention
The invention relates in general to a backlight module and a brightness enhancement film thereof, and more particularly to a brightness enhancement film of a prism structure capable of avoiding the light-leakage to be generated at a large viewing angle.
2. Description of the Related Art
A brightness enhancement film is disposed on a backlight module of a liquid crystal display (LCD) to enhance the light-emitting efficiency. FIG. 1A is a pictorial view showing a conventional brightness enhancement film. Referring to FIG. 1A, a brightness enhancement film 110 includes a main part 120 and several prism structures 130. The prism structures 130 are isosceles right triangular pillars and are repeatedly formed on the main part 120 and arranged in a straight line. FIG. 1B is a graph showing the relationship between the viewing angle and the brightness of the brightness enhancement film of FIG. 1A, wherein the transverse coordinate represents the light outputting angle after the light passes through the conventional brightness enhancement film, and the longitudinal coordinate represents the brightness of light. The thick line in the drawing represents the relationship curve of the viewing angle verse the brightness of the conventional brightness enhancement film in the vertical direction, while the thin line represents the relationship curve of the viewing angle verse the brightness of the conventional brightness enhancement film in the horizontal direction. As shown in FIG. 1B, the light passing through the conventional brightness enhancement film is mostly outputted at the small viewing angle (0° to 45°), so the film has a light gathering effect.
However, a small part of the light encounters the light-leakage phenomenon at a large viewing angle. That is, in FIG. 1B, a part of the light along the horizontal direction is outputted within the range of the viewing angle from 60° to 75°, such that the curve has a local peak value within the range. Because the light emitting region of the large viewing angle (60° to 90°) is a non-effective region, the conventional brightness enhancement film causes the light-leakage in this region, thereby wasting the energy.
There are many technologies relating to the prism structure of the brightness enhancement film being developed. For example, the following methods can achieve the object of enhancing the light-emitting efficiency according to specific technological features.
A brightness enhancement film having several pairs of prism structures is disclosed in U.S. Pat. No. 5,917,664. In each pair of prism structures, two prism angles or two valley angles are the same. This prism structure makes the relationship curve between view angle and brightness partly distribute at a viewing angle smaller than 40°. In addition, when the angle increases, the decreasing trend of the curve is gentler. However, this patent cannot solve the problem of light-leakage at a viewing angle greater than 60° after the light passes through the brightness enhancement film.
An optical film having repeated prism structures is disclosed in U.S. Pat. No. 6,707,611 B2. The optical film can eliminate the problem of luminance non-uniformity. However, this patent also cannot solve the problem of light-leakage at a large viewing angle after the light passes through the optical film.
A brightness enhancement film is disclosed in U.S. Pat. No. 6,280,063 B1. The brightness enhance film includes an upper surface having a specific ratio of an arced prism angle and a lower surface having protrusions for scattering the light. This brightness enhancement film can provide a wider viewing angle in each of the horizontal and vertical directions, and can enhance the light brightness.
Although the above-mentioned prior arts can enhance the light-emitting efficiency, the problem of light-leakage at the viewing angle from 60° to 90° after the light passes through the brightness enhancement film still cannot be solved, and the energy is wasted accordingly.